In the art, it is desirable to produce elastomeric compounds exhibiting reduced hysteresis. Such elastomers, when compounded to form articles such as tires, power belts and the like, will show an increase in rebound, a decrease in rolling resistance and will have less heat build-up when mechanical stresses are applied.
The main source of hysteretic power loss has been established to be due to the section of the polymer chain from the last cross link of the vulcanizate to the end of the polymer chain. This free end cannot be involved in an efficient elastically recoverable process, and as a result, any energy transmitted to this section of the cured sample is lost as heat. It is known in the art that this type of mechanism can be reduced by preparing higher molecular weight polymers which will have fewer end groups. However, this procedure is not useful because processability of the rubber during the addition of compounding ingredients and during shaping operations decreases rapidly with increasing molecular weight.
Another method of reducing hysteresis has been to react a lithium endcapped elastomer with a tin chloride compound to give polymer chains with terminated tin. Tin has an affinity for carbon-black, which affinity reduces hysteresis by removing the effect of a free end. However, with a plurality of polymer chains, endcapping is an inefficient process that results in only about 50-80 percent of the total number of chains being capped with tin. While this method has provided a decrease in hysteresis, the large number of polymer chains without a tin endcap do nothing to reduce hysteresis. In addition, no method is known for endcapping both ends of substantially every polymer chain.
It is also known in the art to employ tin-containing organo-metal compounds as polymerization initiators. For instance, U.S. Pat. No. 3,426,006 discloses the catalyst lithium tributyl tin in diethyl ether for such a purpose. This initiator has been shown by Tamborshi et al, Journal of Organic Chemistry, volume 28, page 237 (1963) to be predominantly an equilibrium mixture of dibutlytin and butyl lithium wherein the butyl lithium is the more active initiator and hence, the polymer chains produced from its initiation actually contain little or no tin atoms. Thus, heretofore, the art has not shown a means whereby substantially each polymer chain of an elastomer can be provided with a tin end group resulting from the initiator.
The present invention provides novel initiators for anionic polymerization, which initiators provide polymer chains having an end group containing a tin atom. The tin containing end group provides the polymer chain with a functional group which can strongly interact with carbon black to (a) provide an elastically efficient end group and (b) greatly improve the dispersability of carbon black throughout the elastomeric composition during compounding. Further, the present invention also provides for polymer chains as above, having another tin atom at the other end of the chains.